The present invention relates to a spread spectrum diversity transmitter/receiver that performs code division multiple access communication by a spread spectrum technology at a digital radio transmission that has a severe multi path fading problem in particular.
At a radio transmission by a fading channel, generally a diversity reception is needed. As fading, there are a flat fading and a selective fading. At the flat fading, a multi path propagation is not generated, but an amplitude/phase of a receiving wave itself is directly varied during the propagation. At the selective fading, the multi path propagation is generated, and an amplitude/phase of arrival waves by each multi path is independently varied. In this case, since the received signal becomes a combined wave of plural multi path waves, depending on a state of the phase shift, the received signal may become an inverse phase combination at a frequency. That is, a frequency selective fade (notch) occurs in a received spectrum. At the flat fading, a variation of a received level is a problem and the received waveform itself is not distorted. However, at the selective fading by the multi path, in addition to the variation of the received level, a distortion of the waveform occurs.
For the fading channel mentioned above, a diversity reception and an adaptive equalizing technology have been conventionally applied. There are several conventional technologies, however in this, as a conventional technology, a spread spectrum communication which is said to be effective against the multi path distortion, is mentioned. The object of the spread spectrum technology is originally developed for military communication being robust against jamming wave. The multi path wave whose delay time is long has a low correlation with a desired main wave signal. In the case that the spread spectrum technology is applied, the correlation between the multi path wave and a spread code can not be established and the multi path wave is suppressed at a de-spread operation. That is, at the spread spectrum technology, the multi path wave is regarded as interference, therefore the spread spectrum technology is a kind of adaptive equalizer.
However, the multi path wave whose delay time is short has a high correlation with a main wave signal, therefore the suppression by the de-spread operation can not be expected. In this case, since the delay time between the multi path wave and the main wave is short, at the time when the relation between the multi path wave and the main wave becomes an inverse phase, a decline of level, that is, a fade out can be generated. In order to cope with this kind of fade out, a diversity reception utilized non-correlation among plural propagation paths becomes indispensable.
FIG. 1 is a diagram showing the structure of a diversity reception. Referring to FIG. 1, the diversity reception is explained. In FIG. 1, the transmission is performed from a transmitter 401 to a receiver 402, radio waves transmitted from the transmitter 401 arrive the receiver 402 via three different paths 403, 404 and 405.
In this, it is assumed that the transmitter 401 transmits the radio waves by using one non-directional antenna. The radio waves emitted from the non-directional antenna are propagated through the path 404 being a direct propagation path, and the paths 403 and 405 through which reflected waves are propagated. By the radio waves emitted from the non-directional antenna are propagated through the different paths, therefore multi path propagation occurs. FIGS. 2A, 2B and 2C are diagrams showing the variation of a received electric field level at each path. In this case, the paths are different in space, the fading generated at each path is independent, and the variations of the received electric field level in the passage of time are shown in FIGS. 2A, 2B and 2C.
In FIG. 2A, the variation of a received electric field level at the path 403 is shown, in FIG. 2B, the variation of a received electric field level at the path 404 is shown and in FIG. 2C, the variation of a received electric field level at the path 405 is shown.
For this kind of propagation, the diversity reception selects or combines the parts not faded out in each diversity branch and makes the probability of fading out decrease. This kind of diversity reception is named as a space diversity or a path diversity because of utilizing the non-correlation among the propagation paths. As a means to realize this diversity, generally an adaptive array using plural antennas is applied. That is, by extracting plural multi path arriving waves using a directional control of the adaptive array, and combining the maximum ratio, a diversity combination can be performed.
However, at the space diversity, plural antennas are needed, therefore a disadvantage at cost occurs. In particular, at microwave communication, the cost of antenna is high and the apparatus becomes large, therefore the number of antennas can not be increased without careful consideration.
In order to improve the problem of this space diversity, Japanese Patent Application Laid-Open No. HEI 8-191289 discloses a spread spectrum diversity transmitter/receiver that utilizes a code division multiplex and a time diversity by a spread spectrum. This conventional spread spectrum diversity transmitter/receiver is shown in FIGS. 3 and 4.
FIG. 3 is a block diagram showing the structure of a transmitting section of this conventional spread spectrum diversity transmitter/receiver, and FIG. 4 is a block diagram showing the structure of a receiving section of this conventional spread spectrum diversity transmitter/receiver.
As shown in FIG. 3, the transmitting section of this conventional spread spectrum diversity transmitter/receiver provides an error correction encoder 101, Mxe2x88x921 pieces of delay element whose delay time is xcfx84M 1031 to 103Mxe2x88x921, M pieces of interleave circuit 1021 to 102M, M pieces of modulator 1051 to 105M, M pieces of spread spectrum circuit 1061 to 106M, a combining circuit 107, a transmitter 108 and a transmitting antenna 109.
The error correction encoder 101 performs an error correction encoding for one series of transmitting data.
The delay elements 1031 to 103Mxe2x88x921, by giving delay time of xcfx84M to an output of the error correction encoder 101 respectively, makes the output of the error correction encoder 101 branch to Mxe2x88x921 pieces.
The interleave circuits 1021 to 102M perform interleave respectively for the output from the error correction encoder 101 and the outputs from the delay elements 1031 to 103Mxe2x88x921.
The modulators 1051 to 105M modulate the outputs from the interleave circuits 1021 to 102M.
The spread spectrum circuits 1061 to 106M perform a spread spectrum operation to the outputs from the modulators 1051 to 105M by different spread codes.
The combining circuit 107 combines the outputs from the spread spectrum circuits 1061 to 106M and performs code division multiplex for them and outputs the result.
The transmitter 108 transmits the code division multiplex signal outputted from the combining circuit 107, through the transmitting antenna 109.
As shown in FIG. 4, the receiving section of this conventional spread spectrum diversity transmitter/receiver provides a receiving antenna 110, a receiver 111, a branching circuit 112, M pieces of de-spread spectrum circuit 1131 to 113M, M pieces of demodulator 1141 to 114M, M pieces of deinterleave circuit 1181 to 118M, M pieces of delay element whose delay time is xcex7N 1161 to 116M, a majority judging circuit 117 and an error correction decoder 119.
The receiver 111 receives the code division multiplex signal transmitted from the transmitting section shown in FIG. 3, through the receiving antenna 110.
The branching circuit 112 makes the signal received at the receiver 111 branch to M pieces and outputs as M branch signals.
The de-spread spectrum circuits 1131 to 113M perform de-spread spectrum operation to the M branch signals outputted from the branching circuit 112 by using the same spread codes used at the time of the spread at the transmitting section.
The demodulators 1141 to 114M demodulate each received signal of the M branch signals performed the de-spread at the de-spread spectrum circuits 1131 to 113M.
The deinterleave circuits 1181 to 118M perform deinterleave respectively for the signals demodulated at the demodulators 1141 to 114M.
The delay elements 1161 to 116M give delay time of xcex7N to each output from the deinterleave circuits 1181 to 118M. In this, the reason why the delay time is given, the delay difference applied to the transmitting section is eliminated and the signal timing of each branch is made to match.
The majority judging circuit 117 performs a majority judgment for each branch signal outputted from the delay elements 1161 to 116M.
The error correction decoder 119 performs an error correction decoding corresponding to the error correction encoder 101 at the transmitting section for the signals outputted from the majority judging circuit 117 and outputs the result as a received signal.
Next, referring to FIGS. 3 and 4, the operation of the conventional spread spectrum diversity transmitter/receiver is explained.
At the transmitting section in FIG. 3, after an error correction is applied to the transmitting data by the error correction encoder 101, this data is made to branch to multi branch of M pieces. And a delay difference is given among branches by the delay elements 1031 to 103Mxe2x88x921, and each branch is used as a time diversity. At the interleave circuits 1021 to 102M, an independent interleave is applied to each branch. After this, at the modulators 1051 to 105M, a modulation is applied to each data. Further, at the spread spectrum circuits 1061 to 106M, a spread spectrum is applied to each data, and at the combining circuit 107, transmitting signals of each branch are combined. In this, each branch is a signal in the same frequency band and the combined signal becomes a code division multiplex signal. The output signal of the combining circuit 107 is converted to a signal of the radio frequency band at the transmitter 108 and transmitted from the transmitting antenna 109.
At the receiving section in FIG. 4, a received signal at the receiving antenna 110 is converted from a signal of the radio frequency band to a signal of the frequency band of the spread spectrum at the receiver 111 and the converted signal is made to branch to M branches at the branching circuit 112. The received signals of M branches made to branch at the branching circuit 112 are inputted to the M pieces of the de-spread spectrum circuit 1131 to 113M corresponding to the transmission section and the de-spread spectrum is applied to the signals. In this operation, the received signals of code division multiplex are extracted every corresponding branches. The extracted received signals are inputted to the M pieces of demodulator 1141 to 114M, and after this are inputted to the M pieces of deinterleave circuit 1181 to 118M and the deinterleave operation is applied. The delay difference applied at the transmitting section is eliminated from the M branch received signals at the delay elements 1161 to 116M, and the signal timing of each branch is matched. The majority judgment is performed for the outputs of the received signals at the majority judging circuit 117, and further the error correction decoding is applied to the outputted signals, at the error correction decoder 119.
At the conventional example in FIGS. 3 and 4, not only a burst error caused by the multi fading is simply made to randomize by the interleave, but the time diversity is applied by the delay difference operation, with this operation, the channel quality against fading is improved. The combining means in the time diversity, for digital signals, finally depends on a switching means. At the conventional example, by applying the majority judgment, more likelihood judgment is performed and the transmission quality is improved.
However, at this conventional example, the combination of the diversity branches is performed by the majority judgment of each branch. For example, at the case that the number of branches is 10 and the three branches show digital signal xe2x80x9c1xe2x80x9d and the remaining seven branches show digital signal xe2x80x9c0xe2x80x9d, the majority judging circuit 117 judges xe2x80x9c0xe2x80x9d. However, at the case that the branches showing xe2x80x9c1xe2x80x9d are five and the branches showing xe2x80x9c0xe2x80x9d are five, there is a problem that the majority judging circuit 117 can not distinguish which judgment is correct. Moreover, since the error probability of each branch is random, in spite of xe2x80x9c1xe2x80x9d is a right signal, there is a possibility that the greater part of the branches are judged as xe2x80x9c0xe2x80x9d. At this case, the majority judging circuit 117 simply judges xe2x80x9c0xe2x80x9d and outputs the result. Therefore at conventional spread spectrum diversity transmitter/receiver, there is a possibility that an error occurs at the majority judgment, and the characteristic of the bit error rate is deteriorated.
Moreover, at this conventional example, the interleave circuits 1021 to 102M and the deinterleave circuits 1181 to 118M are needed at each diversity branch, therefore there is a problem that the size of the apparatus becomes large.
Consequently, at the mentioned above conventional spread spectrum diversity transmitter/receiver, there are following problems:
1) there is a possibility that an error occurs at the majority judgment, at this case that a bit error rate is deteriorated,
2) plural interleave circuits and plural deinterleave circuits are needed at diversity branches, therefore the size of the apparatus becomes large.
It is therefore an object of the present invention to provide a spread spectrum diversity transmitter/receiver whose size is small and in which the characteristic of the bit error rate is improved.
According to the present invention, for achieving the objects, a spread spectrum diversity transmitter/receiver provides a spread spectrum diversity transmitter and a spread spectrum diversity receiver. And said spread spectrum diversity transmitter includes an error correction encoding means for performing an error correction encoding for a series of transmitting data, an interleave means for performing an interleave for a signal outputted from said error correction encoding means, plural delay means which give different delay time for the signal outputted from said interleave means and makes said signal branch to Nxe2x88x921 branches, a convolutional encoding means which performs a convolutional encoding whose coding rate R=N/M for signals of Nxe2x88x921 branches outputted from said plural delay means and for a signal directly outputted from said interleave means, and outputs signals of parallel data of M branches, plural modulating means for modulating said signals of parallel data of M branches outputted from said convolutional encoding means respectively, plural spread spectrum means for performing spread spectrums for signals outputted from said plural modulating means by respective different spread codes, a combining means for performing a code division multiplex by combining said outputs from said plural spread spectrum means, and a transmitting means for transmitting said code division multiplex signal. And said spread spectrum diversity receiver includes a receiving means for receiving said code division multiplex signal transmitted from said spread spectrum diversity transmitter, a branching means for making said signal received at said receiving means branch to M branches and outputs M branch signals, plural de-spread spectrum means for performing de-spread spectrums for said M branch signals by using the same spread codes used at the time when the spread was performed at said spread spectrum diversity transmitter, plural demodulating means for demodulating said M branch signals performed the de-spread at said plural de-spread spectrum means, a Viterbi decoding means to which said demodulated M branch signals are inputted as parallel data and performs a Viterbi decoding whose coding rate R=N/M for said M branch signals demodulated at said plural demodulating means, plural delay means which give different delay time for each branch of parallel data of N branches outputted from said Viterbi decoding means and adjusts each delay, a majority judging means which performs a majority judgment for each branch signal outputted from said delay means and outputs the judged data, a deinterleave means for performing a deinterleave for said judged data at said majority judging means, and an error correction decoding means for performing an error correction decoding corresponding to said error correction encoding means of said spread spectrum diversity transmitter for the data outputted from said deinterleave means.
According to the present invention, at a spread spectrum diversity transmitter/receiver of the present invention, a burst error generated by a short break caused by a multi path fading is corrected by an error correction and an interleave. In addition to this, a diversity transmission and reception is performed by reducing the time correlation among branches by giving delay difference operation, and a convolutional encoding is performed among branches. With this, at the present invention, multi dimensional transmitting signal by code division is utilized not only for diversity itself but also for error correction means. Moreover, by operating a majority judgment that selects a majority among branches, plural diversity branch signals are converted to the most likelihood one series data and a bit error is equivalently corrected. At the spread spectrum diversity receiver, an error correction decoding corresponding to the error correction encoding at the spread spectrum diversity transmitter is performed and the channel quality is improved.
Therefore, at a spread spectrum communication, without using a space diversity or a frequency diversity by installing fixed plural antennas or adaptive arrays, a diversity reception by code division multiplex becomes possible. Therefore, a bit error rate can be improved. And without any relation with the number of branches, it is enough that the number of the interleave circuit and the deinterleave circuit is one each, consequently the size of the apparatus can be reduced.
At another spread spectrum diversity transmitter/receiver of the present invention, an error correction encoding means and an interleave means are deleted from the mentioned above spread spectrum diversity transmitter, and an error correction decoding means and a deinterleave means are deleted from the mentioned above spread spectrum diversity receiver.
At this another spread spectrum diversity transmitter/receiver, only the effect of bit error corrections by the convolutional encoding among branches and the majority judgment is utilized, and the error correction functions and the interleave function are deleted. With this, an error correction additional bit becomes unnecessary due to that the error correction functions are deleted, the signal band can be reduced and the frequency efficiency can be improved.